This invention relates to apparatus which has particular but by no means exclusive application to the determination of the condition or identity of material of biological origin.
It is known to assess the condition of material of biological origin by passing an electrical current through a portion of the material and measuring a selected electrical parameter. Direct, alternating and pulsed current have variusly been proposed and a well known commerical instrument measures the voltage developed across a bi-contact probe (U.S. Pat. No. 3,864,627) by a constant pulsed current.
In a further development of these principles, it is known that for a particular specimen of vegetation, the ratio of the electrical impedance of a fixed length of the vegetation to an applied current of low frequency, e.g. 1 kHz, to the impedance of the same length of the vegetation to a current of high frequency, e.g. 10 kHz, hereinafter referred to as the "impedance ratio", is reduced as the condition of the specimen deteriorates due to stress such as heat or decapitation. With some species of plant, for example, the impedance ratio of a healthy plant is approximately 3:1, whereas the impedance ratio of a dead plant is approximately 1:1.
An instrument for measuringthe impedance at two frequencies and thereby determining the impedance ratio is described in de Plater and Greenham P1 Physiol 34:661-667 (1959). This instrument comprises a wide range AC bridge and its use to determine the impedance ratio has the advantages that such ratio is largely unaffected by moisture content and that when using a probe in homogenous tissue the impedance ratio is independent of the depth of probe insertion; depth of insertion must be kept constant for single reading instruments. However, the bridge instrument requires balancing twice during each determination, a requirment which is inconvenient under field conditions because in the implement of de Plater and Greenham up to 11 controls may require adjustment and the balancing operation may not be entirely objective.
To meet these problems, it has been proposed (Ph.D. thesis by Moore, University of Melbourne 1981) to employ an arrangement in which an AC voltage is applied to the probe and the voltage drop across as reference resistor is measured, that resistor being part of a two-resistor divider of which the plant is the other resistor. However, as a result of using a divider resistor, this arrangement has a hyperbolic response to impedance so that the meter measures impedance in arbitrary and non-linear units.
U.S. Pat. No. 4,408,128 to Fujita describes a moisture meter in which the electrical resistance of a grain or wood sample is measured by applying a DC or very low frequency AC signal (80 Hz) across a probe contactable with the sample, logarithmically scaling the resultant current by means of an operational amplifier having its inputs in series with the probe to produce a display value linearly related to moisture content. It is to be noted that Fujita, in determining moisture content, is concerned strictly with resistance of the sample and hence proposes a DC or 80 Hz AC applied voltage. Biological tissue may be represented by a resistor in parallel with a capacitor. Measurements made by the present applicant indicate ranges of 1 k.OMEGA. to 200 k.OMEGA. for the resistance component and 0.05 nF to 10 nF for the capacitance component. At these values and a frequency of 80 Hz, there would be no effective capacitive contribution to impedance. Fujita moreover relies upon a logarithmic relationship between resistance and moisture content.
In contrast to these features of Fujita, applicant's concern for assessing plant condition or identity requires an examination of both the resistance and capacitance contributions to the impedance and need take no account of any logarithmic relationship with respect to resistance. Furthermore, it is known that the result must be accurate to somewhat better than .+-.5% for statistically significant conclusions to be drawn in regard to tissue condition or identity: such accuracy is not feasible with the circuit configuration of Fujita. To identify differences between biological samples, a statistically significant difference must be shown. This is usually chosen to be at the 95% level although higher levels may sometimes be used. Errors must therefore be substantially less than 5%. Finally, the use of DC voltage as proposed by Fujita is believed unacceptable in biological tissue as it causes irreversible destruction of tissue, even at lower levels.